Receiving Region

Receiving Region
The first region of the alimentary canal
consists of devices for feeding and swallowing.
These include mouthparts (for
example, mandibles, jaws, teeth, radula,
bills), buccal cavity and muscular
pharynx. Most metazoans other than
suspension feeders have salivary
glands (buccal glands) that produce
lubricating secretions containing mucus
to assist swallowing (Figure 34-9). Salivary glands often have other specialized
functions such as secretion of
toxic enzymes for quieting struggling
prey and secretion of salivary enzymes
to begin digestion. The salivary secretion
of the leech, for example, is a complex
mixture containing an anesthetic
substance (making its bite nearly painless)
and several en-zymes that prevent
blood coagulation and increase blood
flow by dilating veins and dissolving
the tissue cement that binds cells
together.

Salivary amylase is a carbohydratesplitting
enzyme that begins hydrolysis
of plant and animal starches. It is found
only in certain herbivorous molluscs,
some insects, and in primate mammals,
including humans. Starches are long
polymers of glucose. Salivary amylase
does not completely hydrolyze starch,
but breaks it down mostly into twoglucose
fragments called maltose. Some free glucose and longer fragments
of starch are also produced. When the
food mass (bolus) is swallowed, salivary
amylase continues to act for
some time, digesting perhaps half of
the starch before the enzyme is inactivated
by the acidic environment of the
stomach. Further starch digestion
resumes beyond the stomach in the
intestine.

The tongue is a vertebrate innovation,
usually attached to the floor of
the mouth, that assists in food manipulation
and swallowing. It may be used
for other purposes, however, such as
food capture (for example, chameleons,
woodpeckers, anteaters) or as
an olfactory sensor (many lizards and
snakes).

Figure 34-10 Oral cavity and throat in humans in sagittal section (A), and sequence of swallowing (B to D).

In humans, swallowing begins with
the tongue pushing moistened food
toward the pharynx. The nasal cavity
closes reflexively by raising the soft
palate. As the food slides into the pharynx,
the epiglottis tips down over the
trachea, nearly closing it (Figure 34-10).
Some particles of food may enter the
opening of the trachea but contraction
of laryngeal muscles prevents it from
going farther. Once food is in the
esophagus, peristaltic contraction of
esophageal muscles forces it smoothly
toward the stomach.